The interaction of ultrasound with biological tissues is a matter of importance both for its possible contributions to an understanding of the equilibrium and kinetic properties of these materials and due to the increasing use of ultrasound as a clinical diagnostic tool. We propose to study the ultrasonic absorption spectra of a variety of proteins and peptides in order to elucidate the relative importance of ionization reactions of side chains, charge redistribution on the surface of the protein, and conformational changes in determining the ultrasonic propagation properties of protein solutions. This will involve the use of new low frequency, small volume ultrasonic measuring techniques in order to extend the frequency range beyond that accessible by current techniques. Theoretical analyses involve the computation of complex relaxation spectra and the computation of the degrees of ionization of individual sites on the protein in order to evaluate the relative contributions of various processes.